Inline layout assertion annotations

In some use-cases, it's valuable to define a format that allows for future expansion; e.g., reserves some number of bytes as "padding", that may then be used by later additions to the format so long as the total size of the record does not change; e.g.:
```rust
#[repr(C)]
#[derive(IntoBytes, FromBytes, KnownLayout)]
struct Abc {
field_a: usize,
field_b: usize,
padding: [u8; 240],
}
```

Right now, customers may use `static_assertions::assert_eq_size` to defend against accidental size changes. In the interest of sparing users from pulling in another dependency, and in the interest of making a best-practice readily available, we might want to consider supporting *inline* size assertions on non-generic types; e.g.:

```rust
#[repr(C)]
#[derive(IntoBytes, FromBytes, KnownLayout)]
#[zerocopy::hint(
align: core::mem::align_of::(),
size: 256,
)]
struct Abc {
field_a: usize,
field_b: usize,
padding: [u8; 240],
}
```